1. Field of the Invention
The present invention generally relates to an encoder device, and particularly relates to an encoder device that detects movement of an object to be measured and generates pulses corresponding to the movement of the object.
2. Description of the Related Art
Encoder devices for generating pulses corresponding to movements of objects to be measured have long been used as devices for detecting linear motion, rotary motion, etc., of objects to be measured and inputting the detection results into digital devices such as computers.
FIGS. 6A and 6B are a plan view and a side view, respectively, each schematically showing a configuration of a related art encoder device. With reference to FIGS. 6A and 6B, the encoder device comprises a light source 10, a slit member 11, and an integrated circuit 12.
The light source 10 is disposed spaced apart from and facing the integrated circuit 12. The slit member 11 is interposed between the light source 10 and the integrated circuit 12, and is movable with respect to the light source 10 and the integrated circuit 12 in an X direction (leftward and rightward). The slit member 11 is fixed to an object to be measured, and moves in the X direction along with motion of the object.
Referring to a plan view of FIG. 7, the integrated circuit 12 includes photodiodes 12a, 12b, 12c, and 12d which are adjacent to each other in the X direction. Each of the photodiodes 12a, 12b, 12c, and 12d has a length of X1 in the X direction and a width of Y1 in a Y direction. Light receiving areas of the photodiodes 12a, 12b, 12c, and 12d are equal to each other.
Referring to a plan view of FIG. 8, the slit member 11 includes light transmitting sections 11a and light shielding sections 11b which are alternately adjacent to each other in the X direction. Each of the light transmitting sections 11a and the light shielding sections 11b has a length of 2×X1 in the X direction and a width of Y1+α (>Y1) in the Y direction.
Referring back to FIG. 6A, output signals of the photodiodes 12a and 12c are compared by a comparator 13, and the comparison result is output as a detection signal from a terminal 14. Output signals of the photodiodes 12b and 12d are compared by a comparator 15, and the comparison result is output as a detection signal from a terminal 16.
When the slit member 11 moves rightward in the X direction with respect to the integrated circuit 12, the incident light intensities on the photodiodes 12a, 12b, 12c, and 12d change as shown in (A), (B), (C), and (D), respectively, of FIG. 9.
Thus, the output signal of the comparator 13 and the output signal of the comparator 15 change as shown in (E) and (F), respectively, of FIG. 9, wherein the waveform of the output signal of the comparator 15 is delayed by a ¼ period relative to the waveform of the output signal of the comparator 13. On the other hand, when the slit member 11 moves leftward in the X direction with respect to the integrated circuit 12, the waveform of the output signal of the comparator 15 is advanced by a ¼ period relative to the waveform of the output signal of the comparator 13.
Incidentally, Japanese Patent Laid-Open Publication No. 6-18290 discloses an encoder device comprising a member that moves in a direction of an array of openings, a light receiving element that detects lights passing through the openings, and a signal processor that generates output pulses corresponding to detection outputs from light receivers of the light receiving element.
As shown in (A) of FIG. 9, when the slit member 11 moves rightward in the X direction relative to the integrated circuit 12 at a constant velocity, the incident light intensity on the photodiode 12a monotonically increases for a time period τ, is maximized for the next time period τ, and then monotonically decreases for the next time period τ. Meanwhile, as shown in (C) of FIG. 9, the incident light intensity on the photodiode 12c monotonically decreases for a time period τ, is minimized for the next time period τ, and monotonically increases for the next time period τ.
The detection signal output from the comparator 13 shown in (E) of FIG. 9 falls at the point when the incident light intensity on the photodiode 12c falls below the incident light intensity on the photodiode 12a. Because of monotonic increase and decrease of the input signals to the comparator 13, a time fluctuation, i.e., jitter on the falling edge of the detection signal output from the comparator 13 is large. For the same reason, jitter on the on the rising edge is also large. Similarly, jitter on the rising and falling edges of the detection signal output from the comparator 15 shown in (F) is large.